Based on the above reference comic, I suppose it's possible that Susan wouldn't care that she possibly has a twin sister.
"Yo, Susan! We found your identical twin!"
"No, really! She's your actual, biological twin sister born twenty minutes before you were! You have a sibling who's living with an adopted family, and you may or may not be adopted yourself!"
"I heard you the first time! Now be quiet. I'm trying to watch Deep Space Nine."
If that winds up actually happening, pretend to act surprised.